Thermal energy storage is a technology that stocks thermal energy by providing thermal or chemical energy to a storage medium so that the stored energy can be used at a later time for heating and cooling applications and power generation. There are three kinds of thermal energy storage systems, namely: sensible heat storage that is based on storing thermal energy by heating or cooling a liquid or solid storage medium (e.g. water, sand, molten salts, rocks); 2) latent heat storage using phase change materials; and 3) thermo-chemical storage using chemical reactions to store and release thermal energy. Thermo-chemical reactions can be used to accumulate and discharge heat and cold on demand in a variety of applications using different chemical reactants. At present, thermal energy storage systems based on sensible heat are commercially available while thermo-chemical reaction and phase change material storage systems are mostly still under development and demonstration.
Energy systems based on hydrogen are interesting candidates to replace fossil fuels for future energy global needs and assist in mitigating the associated environmental problems. The absorption of hydrogen in solid materials and particularly in metal hydrides is a promising technique, due to the high volumetric energy density and the required relatively low pressures.
Currently, electrical energy-based heating and cooling systems are used in electric vehicles because electric vehicles generate insufficient waste heat for heating. In these systems, stored chemical energy is first converted to electrical energy by batteries or fuel cells and subsequently converted into heat. Electrical-to-thermal energy conversion processes such as resistive heating or compressor-based refrigeration are inefficient. In contrast, converting stored chemical energy directly into heat using a thermal energy storage system can be much more efficient. However, thus far, materials for thermal energy storage systems have not had the proper combination of thermodynamic and kinetic properties needed to simultaneous fulfill the energy storage density and power requirements.
The prior art contains numerous examples of metal hydride materials used in a wide variety of thermal energy storage systems. For example, see Sandrock et al., pp. 197-258 in Topics in Applied Physics: Hydrogen in Intermetallic Compounds II, 1992; and Muthukumar and Groll, “Metal hydride based heating and cooling systems: A review, International Journal of Hydrogen Energy 35 3817-3831 (2010). These include systems that utilize MgH2 as part of the hydride material. However, the MgH2 in these systems is used at high temperature, such as 300° C. For example, see Bogdanovic et al., “Active MgH2—Mg systems for reversible chemical energy storage,” Angewandte Chemie 29 223-234, 1990. The MgH2 used in these systems is not suitable for use in vehicle heating and cooling systems because these systems must operate from near room temperature, around 20° C. These systems use alloys that have hydrogen gravimetric capacities of <2.0 wt %, which yield energy densities too low for vehicle applications.
In view of the aforementioned needs in the art, improved thermal energy storage systems employing metal hydrides for reversible hydrogenation and dehydrogenation are desired, preferably addressing both thermodynamics and kinetics requirements of the materials.